Ti3C2 MXenes-based catalysts for the process of α-pinene isomerization

In this study, the catalytic performance of Ti3C2 MXene materials in the reaction of α-pinene isomerization was demonstrated. The influence of etching agents (HF, HF/H2SO4, and HF/HCl; weight ratios of mixed acids: 1 : 3, 1 : 4, and 1 : 5) on removing Al atoms from MAX phase, creation of an accordion-like structure typical for MXenes and catalytic activity of the produced samples have been revealed. The MXene HF obtained by MAX phase HF treatment exhibited the highest activity (conversion of α-pinene 74.65 mol%), while materials produced with the mixed acids (HF/H2SO4 and HF/HCl) showed a significant reduction in the conversion of α-pinene (on average about 28-fold). However, these last samples displayed an increase of about 10 mol% in the selectivity to the most desirable product-camphene. The high activity of MXene HF is a result of a high concentration of acid sites (11.62 mmol g−1) – the concentration of acid sites in the samples obtained by MAX phase mixed acids treatment was about 2.5–5.5 times smaller. This work proposes possible mechanisms for the α-pinene isomerization reaction on the MXene HF and on the MXene HF/H2SO4X : Y and MXene HF/HCl X : Y in connection with their structure.


Introduction
Aer the discovery of graphene in 2004, scientists tried to nd other two-dimensional (2D) materials.In 2011, a novel class of 2D materials termed MXenes attracted great interest from researchers due to their outstanding properties and range of potential applications.The general equation of MXene is M n+1 X n T x , where M is an early transmission metal, X is carbon and/or nitrogen, n = 1, 2, 3 and T x is a surface functional group (-F, ]O, -OH).One of the most popular MXenes is titanium(IV) carbide (Ti 3 C 2 T x ), which resembles layers in its structure. 1i 3 C 2 T x has been tested in multiple application places.For instance, they can be tested as microwave-absorbing materials due to their positive dielectric loss ability.Their excellent exibility and large surface area make them great candidates mainly for catalytic processes and supercapacitors.Moreover, MXenes are potential candidates as electrocatalysts in hydrogen evolution reactions or water splitting.However, in conventional heterogeneous catalysis used in organic chemistry, MXene materials are currently barely investigated. 2ne of the directions of application of MXenes in heterogeneous catalysis may be the isomerization of a-pinenethe terpene compound of natural origin.Three main isomers of pinene occurring in nature were described: a-, band d-pinene (each structural isomer has 2 enantiomeric forms (±)).Among these 3 isomers, a-pinene is the most abundant in nature.It has the smell of pine and is characterized by a fresh, forest aroma.The main source of a-pinene is turpentine, which is obtained from the resin of coniferous trees, mainly pine, by the steam distillation or gasoline extraction of pine stumps.4][5] a-Pinene is of great interest to scientists due to its biologically active properties (it is used as a fragrance in cosmetics, and in medicine for the treatment of respiratory infections or dissolution of kidney stones), as well as due to the possibility of obtaining derivatives of this terpene compound which also have numerous applications, including applications in medicine.The great advantage of a-pinene as the raw material for organic syntheses is that it is a relatively easily available and cheap raw material, and above all reproducible.This means that the syntheses carried out with the participation of a-pinene meet the principles of green chemistry and sustainable development.In addition, the method of isomerization of a-pinene proposed in this work does not require the use of a solvent, which is also benecial for the environment and reduces the cost of this process.
The main a-pinene derivatives that are obtained in the isomerization process are camphene and limonene.Camphene is applied in the syntheses of many chemicals, for example, toxaphene (insecticide), 6 and synthetic camphor, which is an important fragrance, 7 and moreover, the reaction of camphene with guaiacol leads to valuable cyclohexanol products. 8We should also mention camphene applications in medicine, e.g. in the treatment of cancer. 9Limonene also found numerous applications in medicine, cosmetics, and organic synthesis.For example, the mixture of limonene, a-pinene, and 1,8-cineole shows a therapeutic activity similar to the standard antibiotics used in the treatment of respiratory diseases. 10The disproportionation reaction of limonene causes the formation of a mixture of p-cymene and p-menthane. 11Limonene is also converted to (−)-carvone, which is used as the peppermint avor. 12Research is being conducted on the use of limonene as an additive for the production of plastics that will have contact with food (polyethylene, polystyrene, and polylactic acid). 13ther compounds which are formed in smaller amounts in the process of a-pinene isomerization (terpinolene, a-terpinene, gterpinene, and p-cymene) are commercially important compounds widely used in perfume, food, and organic industries and also in medicine. 14n the industrial isomerization of a-pinene, titanium oxide (TiO 2 ) is used as the catalyst.The process is carried out at temperatures above 155 °C for 24 hours.The main products in this process are obtained: camphene, limonene, and tricyclene (the total selectivity of these compounds amounts to 70%). 15In addition, very good results present the work of Arata et al.They used TiO 2 as a catalyst in the a-pinene isomerization process and achieved the conversion of this terpene compound amounted to 93%, at the temperature of 100 °C and for the reaction time of 2 hours but the main product of this process was trans-pinocarveol and its selectivity was 80%. 16The J. E. Sanchez-Velandia et al. obtained NT-TiO 2 -wc catalyst.NT-TiO 2 -wc was characterized by the conversion of a-pinene 99% and the selectivity of camphene 67%, for the reaction time amounted to 45 minutes. 17A major problem associated with the implementation of the process of isomerization of a-pinene on the industrial scale is the deactivation of the heterogeneous catalyst and the long reaction time.Therefore, the search for catalysts that would be characterized by a higher activity was started, which would shorten the reaction time, and at the same time, their use may allow camphene to be obtained with greater selectivity.These new catalysts should be catalysts that can be used many times in the isomerization process and also these materials should be easily regeneratedexamples of such catalysts may be zeolite catalysts (natural origin or synthesized in a laboratory) or materials with a structure similar to the structure of zeolites, which are durable and relatively easy to regenerate.Studies related to the search for new, active catalysts for the a-pinene isomerization process were carried out, among others, with the following heterogeneous catalysts: Ga-SBA-15, 18 PW/SBA-15, 19 Ga-MCM-41, 20 SO 4 /ZrO 4 /HMS, 21 ferrite-type zeolites, 22 acid treated natural zeolites, 23 TCA/Y-zeolite, 24 heattreated natural zeolites 25 natural clays. 26Very good results were obtained by conducting the studies on the natural zeolite-clinoptilolite, which was modied by washing with sulfuric acid solutions of appropriate concentrations (0.01-2 M).In this case, the conducting of the process for a very short time -4 minutes and at the temperature of 70 °C, allowed for the complete conversion of a-pinene, and the selectivity of camphene and limonene amounted to 50 mol% and 30 mol%, respectively. 27 our previous study, 28 multilayered and exfoliated Ti 3 C 2 MXenes are proposed as a new class of catalysts for the reaction of a-pinene isomerization.Here, 100 mol% conversion of apinene in 7 h of process and the selectivity resulting in ∼60 mol% toward camphene formation was revealed.For that time, it was the highest selectivity and conversion reported in the current state of the art.Such promising results motivated us to continue research on Ti 3 C 2 T x MXenes as catalysts in conventional heterogeneous catalysis, especially in the process of a-pinene isomerization.In the present work, Ti 3 C 2 T x MXenes were synthesized by removing aluminum from MAX phase by etching in a mineral acids mixture (HF/HCl and HF/H 2 SO 4 ).Here, the inuence of HF/HCl and HF/H 2 SO 4 weight ratios (1 : 3, 1 : 4, and 1 : 5) on the physicochemical properties and catalytic activity in the isomerization of a-pinene of the produced samples was studied.The obtained results were compared to the sample synthesized by etching Al from MAX phase with 48% hydrouoric acid.

Synthesis of multilayered Ti 3 C 2 T x MXenes
Multilayered Ti 3 C 2 T x MXenes were produced via acidic etching aluminum from MAX phase (Ti 3 C 2 -Al-Ti 3 C 2 -Al-Ti 3 C 2 ).Here, two different etching agents, HF/HCl and HF/H 2 SO 4 with different weight ratios (1 : 3, 1 : 4, and 1 : 5), were used for removing Al atoms from MAX phase.The details of the etching solutions composition and MAX phase quantity are presented in Table 1.Briey, MAX phase was carefully added to an appropriate mixture of acids and stirred for 24 h at room temperature (RT).Aer that, the obtained mixture was diluted with deionized water, and the precipitate was separated via centrifugation.The obtained product was washed with deionized water until the pH was ∼7.Finally, the received sample was dried overnight at 50 °C and stored at 5 °C for further analysis. 28he products were referred to as MXene HF/HCl X : Y and MXene HF/H 2 SO 4 X : Y, where X : Y means the acids weight ratios.Moreover, a reference sample of Ti 3 C 2 T x MXene was also synthesized according to the same procedure as described above but using 48% HF for etching of Al form Ti 3 AlC 2 (see Table 1).The sample was labeled as MXene HF.

Characterization
The morphology of obtained MXenes was characterized via scanning electron microscopy (VEGA3 Tescan).The crystallographic structure of samples was examined via X-ray diffraction (XRD), performed using an AERIS PANalytical X-ray diffractometer with Cu-Ka radiation.Raman spectroscopy (inVia Renishaw; l = 785 nm) was used for the MXene vibrational signal.Thermogravimetric analysis (TGA) was performed in the temperature range between RT and 1400 °C, with a 10 °C min −1 heating ramp and 100 cm −3 min −1 airow on an SDT Q600 thermal analyzer.To determine the specic surface area (SSA) and pore size distribution, we utilized the Brunauer-Emmett-Teller (BET) method, which involved conducting N 2 sorption measurements following a preliminary sample degassing process using the Micrometrics ASAP 2460 system.The X-ray spectroscopy (XPS) measurements were conducted using Mg Ka (hn = 1253.6eV) radiation in a Prevac (Poland) system equipped with a Scienta SES 2002 (Sweden) electron energy analyzer operating with constant transmission energy (E p = 50 eV).The analysis chamber was evacuated to a pressure below 5 × 10 −9 mbar.
The acid site concentration (As) of the samples was measured using the procedure described by Vilcocq et al. 29 Firstly, 20 mg of the appropriate sample was added into 10 cm 3 of 0.01 M NaOH solution and mixed at RT for 4 h.Next, the dispersion was ltered off and the pH of the ltrate was determined by titration using 0.01 M HCl in the presence of phenolphthalein as an indicator.The acid sites concentration was calculated from the formula (1) below: where [OH − ] is the hydroxide molar concentration determined by the titration (mol dm −3 ), V is the volume of NaOH solution added to the studied material (mL) and m is the mass of MXenes (mg).

Isomerization of a-pinene
In the studies on the activity of MXene materials in the isomerization of a-pinene, a 10 cm 3 glass reactor with a reux condenser and magnetic stirrer with heating were used.The reaction mixture consisted of 1 g of a-pinene and 0.05 g of the appropriate catalyst (catalyst content in the reaction mixture amounted to 5 wt%).The reactor was placed in an oil bath and the reaction mixture was stirred at 500 rpm.The isomerization of a-pinene was performed at the temperature of 160 °C and for the reaction time of 6 h.For analysis of the composition of post-reaction mixtures, each post-reaction mixture was centrifuged and the postreaction solution was dissolved in acetone with a 1 : 3 weight ratio.Qualitative analyses of the post-reaction solutions were performed using a GC-MS method with a ThermoQuest apparatus and a DB-5 column (30 m × 0.25 mm × 0.5 mm).The conditions of analyses were as follows: a helium ow rate of 1 mL min −1 , a sample chamber temperature of 200 °C, and a detector temperature of 250 °C.The furnace temperature was held at 50 °C for 2.5 minutes and then increased to 300 °C at a rate of 10 °C min −1 .
Quantitative analyses were performed using a Thermo Electron FOCUS chromatograph and a ZB-1701 column (30 m × 0.53 mm × 1 mm).The conditions of analyses were as follows: the helium ow rate of 1 mL min −1 , the sample chamber temperature of 220 °C, and the detector temperature of 250 °C.The furnace temperature was held at 50 °C for 2 minutes, next it was increased to 100 °C at the rate of 5 °C min −1 , and nally the temperature was increased to 200 °C at the rate of 10 °C min −1 .
The composition of the post-reaction solutions was determined using the internal normalization method, and the mass balance was used to calculate the conversion of a-pinene and appropriate product selectivity (camphene, limonene, a-terpinene, g-terpinene, terpinolene, tricyclene, and p-cymene).The summary selectivity of other products, including small amounts of fenchene, polymeric compounds, and oxidation products, was also calculated.

Results and discussion
SEM images of MAX phase, MXene HF, MXene HF/H 2 SO 4 X : Y, and MXene HF/HCl X : Y are presented in Fig. 1A-I.MAX phase (Fig. 1A) is composed of Ti 3 AlC 2 particles with irregular shapes.The reference MXene HF (Fig. 1B and C) presents a layered structure with clearly marked spacing between nanosheets.Here, a clear difference between the samples obtained aer mixed acids (HF/H 2 SO 4 and HF/HCl) and HF treatment is observed.MXene HF/H 2 SO 4 X : Y samples (Fig. 1D-F) do not show layered structures characteristic of MXenes.Their morphology is similar to the MAX phase (Fig. 1A).Whereas, MXene HF/HCl X : Y (Fig. 1G-I) exhibit a lamellar structure without clearly marked spacing between the nanosheets.It indicates that using a mixture of HF with HCl or H 2 SO 4 with studied weight ratios for removing Al form Ti 3 AlC 2 does not cause the creation of accordion-like structure characteristics for MXenes.Y samples with reference MXene HF, a clear shi of all peaks toward lower angles is detected.It is due to the presence of residual MAX phase in the samples. 32,33As the concentration of HF in the HF/HCl mixture decreases, the intensity of peaks derived from Ti 3 C 2 also decreases, which indicates that the MXenes have not fully developed and give a weaker signal with the same measurement parameters. 34aman spectra of MAX phase, MXene HF, MXene HF/H 2 SO 4 X : Y and MXene HF/HCl X : Y are shown in Fig. 3.The spectrum of MAX phase shows four peaks at ∼201, 269, 631 and 662 cm −1 , which correspond to Ti-Al vibrations. 35MXene HF exhibits ve sharp modes located at ∼126, 207, 364, 621 and 716 cm −1 .The modes observed at ∼126, 364, 621 and 716 cm −1 are related to carbon and titanium vibrations.The peak located at ∼207 and ∼716 cm −1 indicates proper removal of Al from precursor (MAX phase). 36The spectra of MXene HF/H 2 SO 4 X : Y present modes for both MAX phase and MXene.All MXene HF/H 2 SO 4 X : Y show four peaks at ∼123, 202, 367, and 733 cm −1 , as in the case of MXene HF sample.Moreover, the modes located at ∼202 and  Here, it is also seen that all modes have shied values in respect to MXene HF, which means, not fully formed MXene structure. 38The results from Raman are consistent with the results from SEM and XRD.
The acid site concentration (As) of MXene HF, MXene HF/ H 2 SO 4 X : Y, and MXene HF/HCl X : Y is presented in Table 2.The reference MXene HF shows As of 11.62 mmol g −1 .For both MXene HF/HCl X : Y and MXene HF/H 2 SO 4 X : Y.As values are signicantly lower in comparison to MXene HF.For the samples produced via HF/HCl etching, the value of As is kept in the following order: MXene HF/HCl 1 : 3 (4.74mmol g −1 ) > MXene HF/ HCl 1 : 4 (4.65 mmol g −1 ) > MXene HF/HCl 1 : 5 (4.50 mmol g −1 ).It means that the higher HF content in the HF/HCl mixture the higher concentration of acid sites.The acid site concentration for MXene HF/H 2 SO 4 1 : 3, 1 : 4 and 1 : 5 was 2.21, 3.05 and 3.12 mmol g −1 , respectively.Here, an opposite correlation for MXene HF/HCl  Paper RSC Advances X : Y was observed, along with the lower content of HF in HF/ H 2 SO 4 mixture, the higher concentration of acid sites is detected.
In the next stage of the studies, the catalytic activity of MXene HF, MXene HF/H 2 SO 4 X : Y, and MXene HF/HCl X : Y in the reaction of isomerization of a-pinene was examined.The obtained results are presented in Fig. 7.
It is seen from Fig. 7 that taking into account the value of conversion of a-pinene the highest activity showed MXene HF (conversion of a-pinene amounted to 74.65 mol%).All MXene HF/HCl X : Y and MXene HF/H 2 SO 4 X : Y showed a signicant, approximately 28-fold, decrease in activity.At the same time, the samples of MXene HF/H 2 SO 4 X : Y were characterized by a slightly higher activity than the materials of MXene HF/HCl X : Y.The conversion of a-pinene is 4,46, 3.04 and 4.46 mol% for MXene HF/H 2 SO 4 1 : 3, MXene HF/H 2 SO 4 1 : 4, and MXene HF/H 2 SO 4 1 : 5, respectively.On the other hand, the increase of about 8-10 mol% in the selectivity of transformation of apinene to camphene and also the increase of about 2-4 mol% in the selectivity of transformation to tricyclene was detected for the samples synthesized by the treatment with the mixtures of 2 acids.In particular, the increase in selectivity of the transformation to the rst of these compounds is advantageous as camphene is the preferred product of this reaction.It should also be noted that only in the isomerization of a-pinene performed on MXene HF, a-terpinene (selectivity 22.27 mol%) and p-cymene (selectivity 3.88 mol%) were formed.Moreover, a signicant, 20 to 40-fold increase in the selectivity of a-pinene transformation to other products was observed on catalysts which were obtained by the treatment with the mixtures of 2 acids.Other products can be such products as fenchene, polymeric compounds, and also oxidation products.The highest increase in the amount of these compounds was noted for the MXene HF/HCl 1 : 5 (more than 40 times).It is also visible that the lowest selectivity of transformation to camphene was simultaneously obtained on this sample (64.62 mol%)this is the value comparable to the value obtained on the MXene HF.
The above-mentioned observations can be explained by taking into account the structure of MXene HF sample and the samples of catalysts obtained by the treatment with the mixture of 2 acids.For MXene HF, which is characterized by the accordion-like structure with clearly marked spacing between layers, the formation of bicyclic products: camphene and tricyclene, and monocyclic products: a-terpinene and p-cymene, was characteristic.A slight selectivity of the transformation to "other products" (fenchene, polymeric compounds, and also oxidation products) was also observed for MXene HF.In the case of the catalysts obtained by the treatment with the mixture of 2 acids, we observe the formation of only bicyclic products and the increase in the amount of products such as fenchene, polymeric compounds, and also oxidation products.These products are characterized by much larger molecules (oen very spatially expanded), requiring more space for reactions involving them to occur.Looking at the structure of these catalysts (MXene HF/H 2 SO 4 X : Y samples do not show layered structure and MXene HF/HCl X : Y samples exhibit the lamellar structure without clearly marked spacing between the nanosheets) and the MXene HF catalyst structure, it can be assumed that in the case of the MXene HF, the formation of monocyclic products was related to the diffusion of a-pinene molecules into narrow spaces between the layers of this catalyst.Probably the diffusion of a-pinene molecules was difficult and only some of the a-pinene molecules reached there.It should also be taken into account that limitations in the diffusion to the area between the layers may also be related to the presence of anions in these areas (e.g.Cl − and F − ) that were not completely washed out from the catalyst aer synthesis.These anions may additionally limit the diffusion of a-pinene molecules to the area between the layers and block their access to the active sites.Due to the small space between the layers, the formation of monocyclic products and their further transformations (formation of p-cymene) was favored.However, the remaining molecules of a-pinene remained on the surface of the layered catalyst particles and the a-pinene transformation process probably took place on them.Due to much smaller steric constraints on the surface of large, layered MXene HF catalyst particles, it was possible to obtain large, bicyclic products such as camphene and tricyclene.In the case of the MXene HF/H 2 SO 4 X : Y and MXene HF/HCl X : Y catalysts the lack of spatial limitations related to the size of the reaction space (as was for reactions taking place between layers) made it possible to obtain bicyclic products and, at the same time, steric factors did not force the formation of monocyclic products.Also, the formation of fenchene, polymeric products, and oxidation products with higher selectivity is associated with lower steric restrictions on the surface of the catalyst particles.Moreover, due to the signicantly reduced number of acid centers on the surface of particles of MXene HF/H 2 SO 4 X : Y and MXene HF/ HCl X : Y catalysts, the conversion of a-pinene was signicantly reduced.MXene HF was characterized by the concentration of acid sites amounted to 11.62 mmol g −1 , but for MXene HF/HCl X : Y and MXene HF/H 2 SO 4 X : Y, the concentration of acid sites was about 2.5-5.5 times smaller (see Table 2).At the same time, it can also be noted that although the MXene HF/HCl X : Y samples were characterized by a slightly higher concentration of acid sites, their activity in the isomerization of a-pinene was lower than the MXene HF/H 2 SO 4 X : Y materials (taking into account the conversion of a-pinene), which were characterized by almost 2 times greater specic surface area.This difference in the activity of samples obtained by the treatment with the mixtures of two acids may therefore probably be due to the different availability of acid sites on the particles of these materials.Probably in MXene HF/H 2 SO 4 X : Y acid sites are more accessible.
To clarify what functional groups may constitute the active centers on which the a-pinene isomerization reaction takes place, XPS analysis was performed for the material with the highest catalytic activity (MXene HF).The sample overview is presented in Fig. 8A.The results conrmed the presence of Ti 2p, C 1s, O 1s, and F 1s on the surface of the MXene HF.The detailed XPS spectra of Ti 2p (Fig. 8B) demonstrate characteristic peaks at ∼458.1, 459.0, 460.4,462.8, and 464.8 eV which is assigned to Ti 2+ , Ti-O, C-Ti-F x , Ti 2+ , and Ti-O, respectively. 44igh-resolution XPS C 1s spectrum is shown in Fig. 8C.MXene HF exhibits an intense peak located at ∼285.0 eV, which is attributed to the C-C bonds.The material shows two additional peaks at ∼287.7 and 289.7 eV, which correspond to C-O and O-C]O bonds, respectively. 44As depicted in Fig. 8D the spectrum for O 1s is deconvoluted into three components corresponding to the presence of different oxygen species at the surface of MXene HF.The rst peak at ∼532.0 eV is ascribed to Ti-O-H bonding.The second peak at ∼533.8 eV is due to adsorbed H 2 O.The third peak is located at ∼535.0 eV and corresponds to O-F x bonding. 45,46As depicted in Fig. 8E F 1s region is deconvoluted into 2 peaks at ∼688.3, and 690.0 eV, which is attributed to C-F, and F-C-F, respectively. 47he results of tests carried out using the XPS method for the MXene HF catalyst conrmed the presence of Ti-OH groups on the surface of this material, which constitute the active centers of the catalyst (proton source for the a-pinene isomerization reaction).They may, among others, be formed from titanyl groups (Ti]O) in the hydrolysis process.Similarly, C-OH groups present on the surface of the tested catalyst may be the proton source for the isomerization reaction.The C-Ti-F x , O-F x , C-F and F-C-F groups also attract attention.They indicate the binding of uorine in the structure of the tested material, but at the same time, the formation of such groups indicates that protons from HF must also have been bound in some way in the structure.It can be assumed, for example, that HF reacting with the carbonyl group (C]O group) causes the formation of CF(OH) groups, in which the OH group can be the proton source for the isomerization reaction.Similarly, the titanyl group (Ti]O) reacts with HF to form TiF(OH) groups, and here also the OH group can be the proton source for the isomerization reaction.However, a detailed explanation of the mechanism of HF reaction with surface groups and its participation in the formation of acidic centers, which can be the source of a proton for the isomerization reaction, requires further, very detailed research in the future.

Conclusion
Ti 3 C 2 MXenes-based materials were synthesized and applied as catalysts in the reaction of a-pinene isomerization.The impact of the etching medium (HF; HF/HCl and HF/H 2 SO 4 : weight ratios: 1 : 3, 1 : 4, and 1 : 5) on the composition, morphology, and structure of the produced samples was investigated.During the catalytic tests, the highest activity showed MXene HF, the sample with layered structure and clearly marked spacing between nanosheets, which was obtained by the treatment of MAX phase only with HF (conversion of a-pinene achieved a value of 74.65 mol%).The catalytic tests also showed that materials produced by the treatment of MAX phase with HF/ H 2 SO 4 mixtures were slightly more active than the samples obtained by HF/HCl treatment.The latter materials were characterized by the lower concentration of acid sites, but probably these acid sites were more accessible to a-pinene molecules.In general, it can be said that using HF/H 2 SO 4 and HF/HCl mixtures for removing Al form MAX phase led to a signicant reduction in the conversion of a-pinene (on average about 28fold), while increasing by about 10 mol% in the selectivity of the transformation to the most desirable productcamphene.To summarize, further modications of the method of aluminum etching from MAX phase should go towards such a selection of compounds used for etching and their mutual ratio that, while maintaining the increased selectivity of transformation to camphene, signicantly increase the conversion of a-pinene.

Fig. 7
Fig. 7 Results of the catalytic tests for the isomerization of a-pinene for MXene HF, MXene HF/H 2 SO 4 X : Y, and MXene HF/HCl X : Y.

Table 1
30,31sition of the etching solutions and MAX phase quantity for MXenes synthesis 7°, typical for Ti 3 C 2 .30,31AllMXeneHF/H 2 SO 4 X : Y do not show characteristic reections of Ti 3 C 2 .Here, peaks located at 9.5, 19.1, 33.9, 36.7,38.8,41.7, 48.3, 56.3 and 60.1°related to MAX phase are observed (see pattern of MAX phase).It is also seen that MXene HF/H 2 SO 4 X : Y exhibit lower intensity peaks than the MAX phase, which indicates that the structure of the MAX phase has been harmed by the reaction with the acids mixture.The materials synthesized via removing Al from MAX phase by etching in HF/HCl mixture (MXene HF/HCl X : Y) show several reections at 7.7, 18.1, 26.9, 35.4,40.2, 61.0°characteristics of Ti 3 C 2 MXene.However, considering MXene HF/HCl X : © 2023 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2023, 13, 30281-30292 | 30283 Paper RSC Advances 60.

Table 2
Acid site concentrations and specific surface area of obtained samples © 2023 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2023, 13, 30281-30292 | 30287